In flameless absorption spectrophotometer measurements, a sample of material which is to be analyzed is placed in a tubular graphite furnace which is electrically heated so that the sample first dries, then ashes and is eventually heated to the point at which the various elements in the sample become atomized. When the particular element or elements to be measured have been released into their atomic state, a light beam originating from a resonant line-emitting source is passed through the heated furnace tube and into a monochromator and detector, the circuitry of which determines the concentration of the desired element in the sample by measuring the intensities of appropriate portions of the resulting beam after the desired atomic element has absorbed its characteristic resonant lines.
Interferences are observed in atomic absorption spectroscopy whenever the components of the sample matrix alter the atomization time, resonance time and/or atomic population of the elements of interest relative to the atomization of the pure element in the absence of the matrix. Much study has been conducted with respect to chemical environments in which gas phase reactions prevent a species from decomposing to its constituent elements before leaving the optical path. Commercial electrothermal atomizers used in this work, e.g., Varian models CRA-63 and CRA-90 produce strongly attenuated atomic lead signals in the presence of several chloride salts. This is due to an atomizer design which heats the sample-containing atomizer center first. As a result, the analyte element often vaporizes from the furnace surface as a volatile molecule, i.e., PbCl.sub.2, but encounters furnace walls which are either as hot or cooler than the surface from which it left. Therefore, insufficient thermal energy is available for complete decomposition of the molecule to atoms.
The approach to solving the problem of matrix interferences in pulse-heated electrothermal atomizers for Atomic Absorption Spectroscopy (AAS) has recently focused on proper design. Routine analysis by AAS is currently hampered by complex matrices which alter the true elemental sample populations observed by the spectrometer. Matrix-independent atomizers are therefore imperative to any laboratory that requires an increase in production and accuracy. Three main approaches to dealing with this problem are currently in practice:
(1) Chemical modification of the sample involves the use of reagents such as H.sub.3 PO.sub.4 or H.sub.2 gas to remove interfering chlorides during the ash cycle as HCl gas [Czobik et al. (I), Anal. Chem. 50, 2 (1978); Churella et al., Anal. Chem. 50, 309 (1978); Czobik et al. (II), Talenta 24, 573 (1977); Frech et al. (I), Anal. Chim. Acta 82, 83 (1976)], ascorbic and oxalic acids which reduce seawater interference on Co. Cu and Mn [D. J. Hydes, Anal. Chem. 52, 959 (1980)] and many chloride interferences on Pb by a still disputed mechanism [Hydes (supra) and McLaren et al., Analyst 102, 542 (1977)], excess LiNO.sub.3 which binds excess Cl through formation of thermally stable LiCl [B. V. L'vov (I) Spetrochim. Acta 33B, 153 (1978)], and furnaces precoated with lanthanum [Thompson et al., Analyst 102, 310 (1977)] or molybdenum in the presence of phosphoric acid [D. J. Hodges, Analyst 102, 66 (1977)] to overcome various lead interferences in urine and natural waters.
(2) Selective volatilization involves the vaporization and removal of one component of the analyte-matrix system before the other is vaporized. For example, no interference is observed in the Ni-CdCl.sub.2 system since the decomposition products of CdCl.sub.2 leave the furnace before Ni begins to atomize. However, a severe depression of the Pb AA signal is observed for the Pb-CdCl.sub.2 system since the decomposition of CdCl.sub.2 to produce free Cl and the atomization of Pb occur at similar temperatures, therefore Pb and Cl vapor coexist at the same time causing formation of PbCl in the gas phase [Czobik et al. (I), supra]. The relative temporal residence of analyte and inteferent is governed primarily by their respective atomization and decomposition temperatures. However, the extent of overlap can be influenced in some cases by the atomizer length and temperature ramp of the pulse-heated atomizer. Thus, short atomizers and rapid heating will cause rapid diffusion of each species from the atomizer thereby minimizing temporal overlap. It was in this context of vapor phase interference within the atomizer that Czobik and Matousek advocated short furnace designs and short residence times despite the concurrent decrease in the analyte signal obtained [Czobik et al. (I), supra].
(3) The standard additions method assumes an equal fraction of analyte signal perturbation (depression or enhancement) by the matrix for both the analyte level originally present in the sample and added aliquots of standard solutions of the analyte. This technique is commonly used when methods (1) and (2) are unable to produce a matrix independent atomic absorption signal. Whether or not a correct result is obtained depends on the nature of both matrix and analyte.
Although the methods outlined above have provided solutions to some specific problems and in several instances produced insights into the chemistry of some analyte-matrix interaction [Johansson et al., Anal. Chim. Acta 94, 245 (1976); Frech et al. (II), Anal. Chim. Acta 82, 83 (1976)], it has become apparent that the scope of such approaches is limited. For example, multielement atomic absorption becomes impractical when performed with matrix-dependent atomizers since a single chemical pretreatment rarely works for a wide range of elements in one or a complex mixture of matrices; i.e., H.sub.3 PO.sub.4 eliminates interference in the Cu-NaCl system while enhancements are observed in the Pb-NaCl system [Czobik et al. (I), supra]. Use of selective volatilization is limited in scope even for simple systems; i.e., cases where analyte and interferent appear at equal temperatures. This problem becomes even more pronounced when several elements in complex samples are to be measured simultaneously. Standard additions may not work for some elements and, unless the analyst is aware of this, erroneous results are obtained. The increased time and effort expended in determining the effectiveness and utility of the standard additions method is worth PG,6 avoiding, when possible, to any laboratory handling large numbers of samples. In general, laboratory productivity is often reduced by samples with strong matrix interferences when the present matrix sensitive commercial electrothermal atomizers are utilized for routine AAS analysis. When the above "correction" procedures are applied, risk of contamination, erroneous results and analysis time are increased.
The present invention is based upon the conclusion that the best approach is to design an atomizer which produces a matrix independent atomic absorption signal. Ideally, the atoms should be designed to produce an atomization time (.tau..sub.1) which is small (.tau..sub.2 /.tau..sub.1 &gt;1) relative to the average residence time of an atom (.tau..sub.2) in order to minimize variations in peak height and area that arise from matrix induced shifts in the appearance temperature and .tau..sub.1 of the analyte. These requirements are attained with the constant temperature furnaces (CTF) of Woodriff et al. and L'vov [Woodriff et al. (I), Spetrochim. Acta 23B, 665 (1968) and B. V. L'vov (II), Atomic Spectroscopy, Israel Program for Scientific Translations, Jerusalem (1969)]. Unfortunately, in order to reduce size and simplify the design, the commercial electrothermal atomizers which have been developed subsequent to the introduction of the CTF were shortened and pulse-heated. This change in design led to the formation of matrix interferences which were not seen in the original constant temperature furnaces as Hageman et al. [Hageman et al. (I), Anal. Chem. 51, 1406 (1979) and Hageman et al. (II), Appl. Spectr. 33, 226 (1979) demonstrated. However, it was not until 1977 when L'vov proposed his platform that a change in atomizer design was considered as a means of reducing matrix interferences in pulse-heated atomizers. The fact that CTF's are free of matrix effects and that these effects can be reduced or eliminated in pulse furnaces by delaying sample introduction until the atomizer walls and internal gas are hotter than the appearance temperature of the element of interest, suggests that temperature is a key factor. Another factor is the residence time within the hot environment (.tau..sub.2), which is related to length. Thus, ideally, even if analyte-molecule formation due to matrix interactions does occur, decomposition to atoms is complete before the analyte leaves the atomizer. In the case of CTF, such reactions have several seconds to reach completion since the rate of sample travel down the atomizer tube is diffusion controlled. Pulsed atomizers, however, expel the atom cloud from the atomization chamber with expanding inert gas driven by rapidly increasing wall temperature. As a consequence, residence times in most pulsed furnaces are less than 500 milliseconds. Since kinetics probably determine the relative distribution of atomic and molecular species in pulsed atomizers as suggested by Fuller [C. W. Fuller, Analyst 99, 739 ( 1974)] and supported by Holcombe [J. A. Holcombe et al., Anal. Chem. 51, 1205 (1979)], higher temperatures and longer tubes than required for a matrix-free CTF would be expected.
The Varian CRA-63 atomizer is the smallest (9 mm length, 3 mm i.d.) among the commercial electrothermal atomizers currently available. From the viewpoint of selective volatilization, this is advantageous. However, when this is not applicable, some of the most severe matrix interferences are seen, especially for the chlorides. Due to its small internal diameter, it is difficult to utilize a L'vov platform or some modification of it. This prompted a search for an alternate modification of the Varian design which would incorporate the simplicity and effectiveness of the L'vov platform.